Shelter Availability Affects the Size-Dependent Diel Activity of Arctic Charr Salvelinus Alpinus

Boulders, woody debris and other sources of shelter are an important feature of the population ecology of salmonids. For example, shelters provide protection against predators, promote visual isolation, and increase the diversity of micro-habitats and thus food availability. From a behavioral perspective, how shelter availability affects the way individuals allocate their activity in time (i.e., diel activity) is rarely studied. Here, we designed a field study using stream enclosures with either high or low shelter availability during summertime in an Icelandic stream. We monitored 1+ Arctic charr and collected repeated measurements of activity eight times of the day (every three hours). In shelter limited enclosures, fish increased their overall activity, became diurnal and were active for longer periods, compared to fish with access to abundant shelters. In addition, body mass affected the allocation of activity in time, albeit only in shelter rich enclosures. Growth rates were similar across treatments, but fish exhibited increased schooling behavior, and reduced foraging rates at low shelter availability. Overall, individuals adopted riskier and less efficient foraging strategies when shelters were limited, which highlights the need to consider the temporal behavior of salmonids in river management policies.